1. Field of the Invention
The present invention relates to a load determining apparatus. More precisely, the present invention relates to an apparatus that uses acoustic or sound waves passed through a structure under load, which load causes a change in the velocity of the acoustic wave that is detected and then converted into an estimate of the magnitude of the applied load.
2. Description of the Prior Art and Related Information
How well a structure endures forces, torques, moments, stresses and strains determines how well it will perform under real life conditions. Hence, it is critical that these loads be closely monitored.
One way of monitoring or measuring load is through strain gage technology. Generally speaking, a strain gage is constructed by embedding a grid pattern of wires or metal foil into a block of resin. The resin block is then cemented to a structure which is to be placed under load and analyzed. Loads such as forces, torques, moments, etc. acting on the structure cause it to distort; and because the resin block is tightly cemented to the structure, distortion takes place therein as well. Distortion of the resin block also bends, stretches or compresses the wires inside. The bending, compressing and stretching change the resistance of the wires, which resistance is easily measured and then converted into a quantity representing stress.
Stress is defined as force acting over an area, so multiplying the measured stress by the affected area results in an estimation of the amount of force. Other loads such as bending moments and torques that equate to a force acting over a bending arm may be similarly estimated.
But strain gages have numerous inherent shortcomings. First, strain gages rely on Hooke's law to determine the amount of elastic strain in a given structure. Hooke's law is expressed as .sigma.=K.multidot..epsilon., where .sigma. represents stress, .epsilon. is strain, and K is the Modulus of Elasticity. Importantly, when Hooke's law is applied to strain gages, the relationship between stress and strain is linear over only a small range. Aggravating the non-linearity problem is that strain gages have their own inherent non-linearity so that even when measuring strain within the linear range of the stress-strain curve, many calibration points must be established to ensure accuracy. Of course, once the measured strain exceeds the linear range of the stress-strain curve, the strain gage no longer functions accurately.
Second, the accuracy of strain gage measurements is severely reduced by the tendency of the measured strain to lag with the actual change in stress. Therefore, during periods of increasing actual stress, the measured stress is lower than for the same stress measured during periods of decreasing stress. This phenomenon is somewhat analogous to a hysteresis effect, wherein the measuring instrument, the strain gage here, does not return to zero after unloading.
Third, strain gages tend to yield after being subjected to stress for a period of time. The effect of this yielding is that the measured stress decreases with time. For that reason, strain gage systems require frequent recalibration. Like its counterpart in metals, this phenomenon is sometimes called creep.
Another conventional stress measurement device is a piezoelectric effect load cell. Such a device operates under the piezoelectric effect theory, well-known in the art. Unfortunately, the piezoelectric effect load cell has many of the shortcomings already discussed in connection with the strain gage.
Accordingly, a need presently exists for a device that measures a given load, but does not exhibit the non-linearity, hysteresis and creep problems associated with strain gages or piezoelectric effect load cells. Such a device would improve the accuracy of load measurement in physical structures.